Flow Cytometric immunophenotyping is a sensitive technique for analysis of benign and malignant tumors. We are studying the refinement of this technique and its application to diagnosis and measurement of prognostic markers in different systems. CD2 is expressed by T and natural killer (NK) cells. Although CD2+ B-cells have been identified in normal fetal and post-natal thymus, they had not been reported in adults. We retrospectively reviewed consecutive low grade B-cell leukemias and lymphomas to investigate the frequency of CD2 expression. CD2 coexpression was observed in 13/83 (16%) CLL, 16/29 (55%) follicle center lymphoma (FCL), 3/12 (25%) hairy cell leukemia (HCL), 0/6 mantle cell lymphoma (MCL), 8/28 (29%) of large cell lymphoma (LCL) and 0/5 marginal zone/MALT lymphoma (MZL/MALT), indicating it is a more prevalent phenomena than previously appreciated. We also reviewed samples from normal healthy donors and determined that 5.74 +/- 2.46% (mean +/- S.D.) of peripheral blood B-cells and 6.48 +/- 1.62 % (mean +/- S.D.) of bone marrow B cells co-express CD2. These data demonstrate that CD2 expressing B-cells are not confined to the thymus, as previously thought, and represent a normal cellular population within the peripheral blood. Furthermore, CD2 can be expressed by low grade B-cell leukemias or lymphomas, and most likely represents a clonal proliferation of this naturally occurring B-cell population. The laboratory has an ongoing interest in detection of minimal and residual lymphoma by using multiparametric approaches to improve the sensitivity of detection of monoclonal B-cell populations. We studied 86 specimens from 24 patients with hairy cell leukemia to determine the sensitivity of routine flow cytometry and consensus primer PCR in this disease. Flow cytometry was more sensitive, detecting hairy cell leukemia in 48/86 specimens (56%), while clonal B cell populations were detected by consensus primer PCR in only 23/86 (27%) specimens. Consensus primer PCR positivity is associated with higher tumor cell numbers as determined by flow cytometry (p=0.0017). We found that flow cytometry is superior to consensus primer PCR in detecting minimal residual hairy cell leukemia. It is more sensitive, more specific and allows quantitation of tumor cell number.